This invention has particular application to paper box folding and gluing apparatus that move blanks along a paper line to form open-ended boxes with a longitudinal seam as an intermediate packaging stage. Typically the seam is formed by simply overlapping the longitudinal edges of a blank coated with a thermoplastic material. The edges are locally heated, overlapped, and sealed by pressing them together while they cool to form the seal. Alternatively the thermoplastic may be replaced by some form of adhesive that is applied to the edges prior the sealing operation.
Such apparatus have utilized hot air directed by nozzles onto selected portions of each blank to activate a thermoplastic resin or other material prior to mechanically forcing the edges together form a seal. Such apparatus are described in the following U.S. Letters Patents:
______________________________________ U.S. Pat. No. Applicant Issued ______________________________________ 3,511,139 Edkvist May 12, 1970 3,562,920 Vuilleumier et al. February 16, 1971 3,587,411 Theys et al. June 28, 1971 3,847,540 Farfaglia et al. November 12, 1974 4,252,052 Meyers et al. February 4, 1981 ______________________________________
Other apparatus convert flat blanks into tubular or rectangular units by heating a thermoplastic resin with a nozzle apparatus that does not contact the materials. The heating occurs just prior to a sealing operation. Such apparatus are described in the following U.S. Letters Patents:
______________________________________ U.S. Pat. No. Applicant Issued ______________________________________ 3,597,900 Scott August 10, 1971 3,751,876 Oakley August 14, 1973 ______________________________________
In many applications, particularly those involving the storage of consumable liquids, such single seals formed by a pair of overlapped edges, such as shown in the foregoing U.S. Letters Patent, are not sufficiently reliable and are prone to leakage.